For transmission systems for synchronous communications, for example, technologies have been known for connecting transmission devices by a redundant configuration using an active line and a spare line and, when a failure occurs in the active line, making a connection to the spare line by uninterruptible switching from the active line to the spare line. Furthermore, in recent years, also for transmission systems for asynchronous communications, there has been a demand for technologies for making a connection to a spare line by uninterruptible switching from an active line to the spare line. Transmission systems for asynchronous communications need a system for phasing the signal received by the active line and the signal received by the spare line.
There is therefore a method of transmitting a signal attached with a sequential number (SN) in a protection section between transmission devices in order to measure the signal delay difference (phase difference) between an active line and a spare line between the transmission devices in a transmission system by using the same signal. In this case, a method of storing an SN in a payload filed in a signal is employed as a method of attaching an SN to a signal. Accordingly, the transmission device refers to the SN in the signal from the active line and the SN in the signal from the spare line and absorbs the delay difference by phasing the signals in accordance with the phase difference between the signals from the active line and the spare line having the same SN.
International Publication Pamphlet No. WO 2006/001060
A transmission device has to secure a payload filed in a signal into which a SN is inserted and then transmit the signal to an opposing device; however, when there is no free space in the payload filed in the signal, e.g., when the traffic of approximately 100% is input and then the SN is inserted into the signal, a line overflow occurs and thus a signal loss occurs.